Artifact correction and source analysis of early electroencephalographic responses evoked by transcranial magnetic stimulation over primary motor cortex
Section snippets
Experimental procedures
A total of 7 healthy volunteers (2 female, 2 left handed, age 28 ± 3) participated in the study after giving a written informed consent. The study protocol was approved by the Ethics Committee of Helsinki Central University Hospital.
Biphasic magnetic pulses (duration 385 μs) were delivered with a custom-made figure-of-eight coil consisting of two coplanar circular loops (40 mm Ø) so that the induced current was in the posterior–anterior direction. The electromyogram was recorded bilaterally from
Correction of the residual artifact
The data was recorded with an artifact blocking sample-and-hold circuit. Fig. 2A illustrates the dependence of the magnitude of the residual artifacts not blocked by this circuit on stimulus intensity. There was a general trend for increase in artifact magnitude with TMS intensity. However, at the same time the artifact amplitude varied greatly between subjects and in one case high artifact amplitudes were observed even for relatively low TMS intensities. For the specific case of average
Discussion
In the present study we showed that a previously published artifact correction method (Berg and Scherg, 1994) makes it possible to almost completely remove the TMS-induced artifact and reveal physiological EEG responses to TMS, starting as early as 10 ms after the magnetic stimulus. We applied the artifact correction to responses evoked by magnetic stimulation of the left and right M1. Based on the corrected data we suggest a model for the generation of these responses and their dependence on
Acknowledgments
VL was funded by Technion stipend for graduate students, by the German Academic Exchange Service (DAAD) and by Boehringer Ingelheim Fonds. We thank Robert Oostenveld and Eric Maris for their assistance with nonparametric statistical analysis and Naomi Bleich for graphical work.
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